Calibration of X-ray reflectometry system

ABSTRACT

A method for inspection of a sample includes irradiating the sample with a beam of X-rays and measuring a distribution of the X-rays that are emitted from the sample responsively to the beam, thereby generating an X-ray spectrum. An assessment is made of an effect on the spectrum of a non-uniformity of the beam, and the spectrum is corrected responsively to the effect.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.10/313,280, U.S. patent application Ser. No. 10/364,883, and U.S. patentapplication Ser. No. 10/689,314, which are assigned to the assignee ofthe present patent application, and whose disclosures are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to analytical instruments, andspecifically to instruments and methods for thin film analysis usingX-rays.

BACKGROUND OF THE INVENTION

X-ray reflectometry (XRR) is a well-known technique for measuring thethickness, density and surface quality of thin film layers deposited ona substrate. Such reflectometers typically operate by irradiating asample with a beam of X-rays at grazing incidence, i.e., at a smallangle relative to the surface of the sample, in the vicinity of thetotal external reflection angle of the sample material. Measurement ofX-ray intensity reflected from the sample as a function of angle gives apattern of interference fringes, which is analyzed to determine theproperties of the film layers responsible for creating the fringepattern.

U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is incorporatedherein by reference, describes an X-ray thickness gauge based onreflectometric measurement. A curved, reflective X-ray monochromator isused to focus X-rays onto the surface of a sample. A position-sensitivedetector, such as a photodiode detector array, senses the X-raysreflected from the surface and produces an intensity signal as afunction of reflection angle. The angle-dependent signal is analyzed todetermine properties of the structure of a thin film layer on thesample, including thickness, density and surface roughness.

U.S. Pat. Nos. 6,512,814 and 6,639,968, to Yokhin et al., whosedisclosures are incorporated herein by reference, describe an X-rayreflectometry system that includes a dynamic shutter, which isadjustably positionable to intercept the X-rays incident on the sample.This shutter, along with other features of the system, permits detectionof XRR fringe patterns with high dynamic range. These patents alsodisclose improved methods for analysis of the XRR fringe pattern inorder to determine thin film properties, including density, thicknessand surface roughness. The high dynamic range enables the system todetermine these properties accurately not only for the upper thin filmlayer, but also for one or more underlying layers on the surface of thesample.

U.S. Patent Application Publication US 2004/0052330 A1, to Koppel etal., whose disclosure is incorporated herein by reference, describesmethods for calibration and alignment of an XRR system for measuringthin films. One such method involves accurately determining, for eachsample placement, the pixel number at which the extended plane of thesample intercepts the detector array that is used in the XRRmeasurements. The incident X-ray intensity corresponding to each pixelis used in making an amplitude calibration of the system. Thispublication also describes a method for aligning an angle-resolved X-rayreflectometer that uses a focusing optic and validating the focusingoptic. In addition, the publication describes methods for correction ofmeasurement errors caused by the tilt or slope of the sample andcalibration of the vertical position of the sample.

XRR may also be used in situ, within a deposition furnace, to inspectthin film layers in production on a semiconductor wafer, as described,for example, by Hayashi et al., in U.S. Patent Application PublicationUS 2001/0043668 A1, whose disclosure is incorporated herein byreference. The furnace is provided with X-ray incidence and extractionwindows in its side walls. The substrate upon which the thin film hasbeen deposited is irradiated through the incidence window, and theX-rays reflected from the substrate are sensed through the X-rayextraction window.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide devices and methods forcalibration of a system for X-ray spectroscopy, such as an XRR system.In these embodiments, an X-ray source irradiates a sample with a beam ofX-rays, which is typically focused to a small spot or line on thesurface of the sample. A detector array measures the intensity of theX-rays emitted from the surface as a function of angle. In the case ofXRR, the X-rays are emitted by specular reflection from the surface, andeach detector element in the array receives X-rays at a different angleof reflection. Thus, the system measures an X-ray spectrum of thesample, but the accuracy of this spectrum may be compromised byimperfect alignment of system components and lack of uniformity in theincident X-ray beam. Precise calibration is needed in order tocompensate for these flaws. The embodiments described hereinbelow permitthe system to be calibrated with enhanced precision and greater speedthan methods known in the art.

In some embodiments of the present invention, an XRR system comprises abeam-limiting optic, such as a knife edge, which is positioned close tothe surface of a sample in order to enhance measurement accuracy. Formeasurements at low incidence angles, the knife edge is lowered verynear to the surface, intercepting the incident X-ray beam and thusshortening the lateral dimension of the spot on the surface. Use of theknife edge in this manner improves the spatial resolution (with respectto the surface of the sample) of the XRR measurements, particularly atlow angles at which the lateral dimension of the spot is most greatlyelongated. Due to non-uniformity in focusing of the X-ray beam onto thesurface, however, there are typically non-uniform variations in the beamintensity as a function of angle, which change when the knife edge isinserted into the beam. The non-uniformity tends to reduce the accuracyof measurement of the XRR spectrum of the sample. Methods are providedfor measuring and compensating for this non-uniformity and itsdependence on the knife position.

In addition, inserting the knife edge into the X-ray beam may shift thelocation of the focal point of the incident beam (i.e., the location ofthe center of mass of the focal spot on the surface). In an embodimentof the present invention, this shift in the focal point location ismeasured. Based on this measurement, the position of the detector arrayand/or of the knife edge is adjusted when the knife edge is used inorder to maintain a constant distance between the focal point and thedetector array. The variations in beam uniformity and focal spotposition may be mapped in advance as a function of the knife edge, andthen used in correcting subsequent measurements made on actual samples.

In another embodiment of the present invention, the orientation of thedetector array is adjusted relative to the surface of the sample. Foroptimal angular resolution, it is desirable that the axis of the arraybe precisely perpendicular to the sample surface. A method is thereforeprovided for aligning the array axis based on signals received from thearray. Additionally or alternatively, the tilt angle of the samplesurface may be measured and, optionally, mapped (as described below),and the known tilt angle may then be used to align the array, as well asthe knife edge. Further additionally or alternatively, the measured tiltangle may be used to correct the angular scale of the X-ray spectracaptured by the detector array.

In some embodiments of the present invention, the sample comprises athin, flat structure, such as a semiconductor wafer, which is held on amounting assembly, such as a chuck or motion stage, during XRRmeasurement. The mounting assembly shifts the position of the samplerelative to the X-ray source and detector so that XRR measurements maybe made at multiple different positions on the surface. Typically, thesurface angle of the sample in the mounting assembly is not perfectlyuniform over the entire surface of the sample. Accurate XRR measurementrequires that the tilt angle at each point be known and taken intoaccount. Measuring the surface tilt at all measurement points on thesample surface, however, is time-consuming. To mitigate this problem,the tilt is mapped in advance as a function of position using areference surface mounted on the mounting assembly. The tilt may bemeasured using the novel methods described herein or using any othersuitable method known in the art. Typically, the measurements arerepeated over a number of reference surfaces and then averaged in orderto eliminate spurious variations. The tilt map of the reference sampleis used to calculate the tilt angles at all measurement points on thesample surface by interpolation.

Although the embodiments described herein are directed primarily to XRRmeasurement, the principles of the present invention may also be appliedin calibration of other radiation-based systems for analysis ofmaterials and thin film measurements.

There is therefore provided, in accordance with an embodiment of thepresent invention, a method for inspection of a sample, including:

irradiating the sample with a beam of X-rays;

measuring a distribution of the X-rays that are emitted from the sampleresponsively to the beam, thereby generating an X-ray spectrum;

assessing an effect on the spectrum of a non-uniformity of the beam; and

correcting the spectrum responsively to the effect.

In one aspect of the invention, measuring the distribution includesmeasuring the distribution of the X-rays that are reflected from thesample as a function of an elevation angle relative to a surface of thesample. In disclosed embodiments, irradiating the sample includesfocusing the beam so that the X-rays converge on the sample over a rangeof incidence angles. Typically, focusing the beam includes forming afocal spot on the surface of the sample, and assessing the effectincludes assessing a variation in a location of the focal spot. In oneembodiment, measuring the distribution of the X-rays includes receivingthe reflected X-rays at a detector, and correcting the spectrum includesadjusting a position of the detector responsively to the variation inthe location of the focal spot. Adjusting the position may includealigning the detector so as to maintain a constant distance between thefocal spot and the detector notwithstanding the variation in thelocation of the focal spot.

Additionally or alternatively, irradiating the sample includesintroducing a beam-limiting optic into the beam, and assessing theeffect includes finding the variation in the location due tointroduction of the beam-limiting optic. In disclosed embodiments,introducing the beam-limiting optic includes positioning a knife edge soas to intercept the beam in a position adjacent to the focal spot.Typically, positioning the knife edge includes adjusting a position ofthe knife edge, and finding the variation in the location includesmeasuring the variation as a function of the position of the knife edge.Additionally or alternatively, adjusting the position includes adjustinga height of the knife edge relative to the surface of the sample, andcorrecting the spectrum includes determining a correction to apply tothe spectrum responsively to the height. Further additionally oralternatively, adjusting the position includes adjusting a lateralposition of the knife edge relative to the beam, and wherein correctingthe spectrum includes selecting the lateral position so as to minimizethe effect of the non-uniformity.

In some embodiments, measuring the distribution of the X-rays includesrecording measurement values as a function of the elevation angle, andcorrecting the spectrum includes modifying the measurement values toaccount for the variation in the location of the focal spot. In one ofthese embodiments, assessing the variation includes determining aneffective variation in the location of the focal spot as a function ofthe elevation angle, and modifying the measurement values includesadjusting a mapping of the measurement values to elevation anglesresponsively to the effective variation.

In disclosed embodiments, irradiating the sample includes introducing abeam-limiting optic into the beam, and assessing the effect includesmeasuring a variation in the beam as a function of the elevation angledue to introduction of the beam-limiting optic. Typically, irradiatingthe sample includes directing the beam to impinge on the sample at afocal location, and introducing the beam-limiting optic includespositioning a knife edge so as to intercept the beam in a positionadjacent to the focal location. In one embodiment, positioning the knifeedge includes adjusting a height of the knife edge relative to thesurface of the sample, and correcting the spectrum includes determininga correction to apply to the spectrum responsively to the height.Additionally or alternatively, positioning the knife edge includesadjusting a lateral position of the knife edge relative to the beam, andcorrecting the spectrum includes selecting the lateral position so as tominimize the effect of the non-uniformity.

In another aspect of the invention, assessing the effect includesdetermining a correction factor as a function of the elevation angleresponsively to the measured variation, and correcting the spectrumincludes applying the correction factor to the spectrum. In a disclosedembodiment, determining the correction factor includes directing thebeam toward a detector array including a plurality of detector elements,making a first measurement of a flux of the X-rays that is incident oneach of the detector elements with the beam-limiting optic in a firstposition, making a second measurement of the flux of the X-rays that isincident on each of the detector elements with the beam-limiting opticin a second position, and comparing the first and second measurements inorder to determine the correction factor.

Typically, measuring the distribution of the X-rays that are reflectedfrom the sample includes measuring the distribution using the detectorarray with the beam-limiting optic in the first position, and making thefirst and second measurements includes removing the sample from the beamof X-rays so that the beam is directly incident on the detector array.Additionally or alternatively, making the first and second measurementsincludes introducing a reflective surface into the beam at a location ofthe sample so that the X-rays are reflected onto the detector array.

In a further aspect of the invention, irradiating the sample includesintroducing a beam-limiting optic into the beam, and assessing theeffect includes measuring a variation in the beam due to introduction ofthe beam-limiting optic. In some embodiments, assessing the effectincludes determining a correction vector responsively to the measuredvariation, and correcting the spectrum includes applying the correctionvector to the spectrum. In a disclosed embodiment, determining thecorrection vector includes directing the beam toward a detector arrayincluding a plurality of detector elements, making a first measurementof a flux of the X-rays that is incident on each of the detectorelements with the beam-limiting optic in a first position, making asecond measurement of the flux of the X-rays that is incident on each ofthe detector elements with the beam-limiting optic in a second position,and comparing the first and second measurements in order to determinethe correction vector. In one embodiment, the detector array has anaxis, and the method includes rotating the array so as to position theaxis perpendicular to a surface of the sample.

There is also provided, in accordance with an embodiment of the presentinvention, a method for inspection of a sample, including:

irradiating a surface of the sample with a beam of X-rays;

measuring a distribution of the X-rays that are emitted from the sampleresponsively to the beam using a detector array, which has an axis andincludes a plurality of detector elements arranged along the axis; and

rotating the detector array so as to position the axis perpendicular tothe surface of the sample.

In disclosed embodiments, measuring the distribution includes measuringthe distribution of the X-rays that are reflected from the sample as afunction of an elevation angle relative to a surface of the sample.Typically, measuring the distribution of the X-rays includes observingan oscillatory pattern in the X-rays emitted as a function of theelevation angle, and rotating the detector array includes aligning thedetector array responsively to the oscillatory pattern. In oneembodiment, aligning the detector array includes rotating the detectorarray so as to maximize a contrast of the oscillatory pattern.

Additionally or alternatively, irradiating the surface includesdirecting the beam toward the surface at a grazing incidence, andmeasuring the distribution includes detecting a transition in thedistribution corresponding to a plane of the surface of the sample, androtating the detector array includes aligning the detector arrayresponsively to the transition. Typically, aligning the detector arrayincludes rotating the detector array so as to maximize a sharpness ofthe transition.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for inspection of a sample, including:

providing a tilt map, indicating a characteristic tilt angle of areference surface at multiple points on the reference surface;

acquiring an X-ray reflectance (XRR) spectrum at a location on a sample;

determining an estimated tilt angle of the location on the sample basedon the tilt map; and

correcting the XRR spectrum responsively to the estimated tilt angle.

In a disclosed embodiment, providing the tilt map includes measuring thecharacteristic tilt angle of the reference surface at each of themultiple points. Typically, acquiring the XRR spectrum includes mountingthe sample on a mounting assembly, and measuring the characteristic tiltangle includes mounting the reference surface on the mounting assembly,and measuring the characteristic tilt angle while the reference surfaceis on the mounting assembly. In one embodiment, measuring thecharacteristic tilt angle includes rotating and translating the mountingassembly, and taking measurements of the tilt angle as a function ofrotation and translation.

Additionally or alternatively, determining the estimated tilt angleincludes finding the estimated tilt angle by interpolation along a curvein the tilt map.

In disclosed embodiments, the sample includes a semiconductor wafer.

There is further provided, in accordance with an embodiment of thepresent invention, a method for inspection of a sample, including:

focusing a beam of X-rays onto a focal location on the sample;

positioning a knife edge so as to intercept the beam in a positionadjacent to the focal location;

measuring a distribution of the X-rays that are emitted from the sampleresponsively to the beam and to the position of the knife edge; and

adjusting the position of the knife edge responsively to thedistribution.

In one aspect of the invention, adjusting the position includesadjusting a lateral location of the knife edge relative to the beam.Typically, measuring the distribution includes measuring a variation inthe distribution of the X-rays as a function of a vertical location ofthe knife edge relative to a surface of the sample at each of aplurality of lateral locations of the knife edge, and adjusting thelateral location includes choosing the lateral location responsively tothe variation in the distribution. In a disclosed embodiment, choosingthe lateral location includes finding the lateral location thatminimizes the variation in the distribution as a function of thevertical location.

Additionally or alternatively, adjusting the position includes adjustinga skew angle of the knife edge relative to a surface of the sample.

There is moreover provided, in accordance with an embodiment of thepresent invention, a method for inspection of a sample, including:

irradiating the sample with a beam of X-rays over a range of angles ofincidence;

positioning a shutter so as to intercept the beam at a predeterminedangle;

measuring a distribution of the X-rays that are emitted from the sampleresponsively to the beam, thereby generating an X-ray spectrum, whichincludes a shadow of the shutter;

determining a tilt angle of the sample responsively to an angularposition of the shadow in the spectrum; and

processing the spectrum responsively to the tilt angle.

In one embodiment, measuring the distribution of the X-rays includesmeasuring the distribution of the X-rays that are reflected from thesample as a function of the elevation angle relative to a surface of thesample, and processing the spectrum includes calibrating the spectrumwith respect to the tilt angle. Typically, the predetermined angle isbelow a critical angle of the sample for total external reflection.

In another embodiment, measuring the distribution includes finding acurrent angular position of the shadow, and determining the tilt angleincludes comparing the current angular position to a reference angularposition of the shadow, which is indicative of a zero tilt angle.Typically, comparing the current angular position to the referenceangular position includes finding a difference between the current andreference angular positions, and determining the tilt angle of thesample to be equal to half the difference.

There is furthermore provided, in accordance with an embodiment of thepresent invention, a method for inspection of a sample, including:

focusing a beam of X-rays onto a focal location on the sample;

measuring a distribution of the X-rays that are reflected from thesample responsively to the beam, thereby generating an actualreflectance spectrum;

estimating a spot size of the beam on the sample at the focal location;

computing a simulated reflectance spectrum of the sample responsively tothe spot size; and

fitting the simulated reflectance spectrum to the actual reflectancespectrum in order to determine one or more properties of the sample.

In one aspect of the invention, computing the simulated reflectancespectrum includes blurring the simulated reflectance spectrum based onan angular spread of the reflected X-rays due to the spot size, as wellas due to the detector vertical pixel size and inaccuracy in alignment.In a disclosed embodiment, estimating the spot size includes assessing avariation in an effective spot size of the beam as a function of anelevation angle relative to the sample, and blurring the simulatedreflectance spectrum includes applying a variable blur to the simulatedreflectance spectrum responsively to the variation in the effective spotsize.

In another aspect of the invention, focusing the beam of X-rays includespositioning a beam-limiting optic in the beam, and estimating the spotsize includes determining the spot size as a function of a position ofthe beam-limiting optic relative to a surface of the sample.

In a disclosed embodiment, fitting the simulated reflectance spectrum tothe actual reflectance spectrum includes determining at least one of athickness, a density and a surface quality of a surface layer of thesample.

There is also provided, in accordance with an embodiment of the presentinvention, apparatus for inspection of a sample, including:

an X-ray source, which is adapted to irradiate the sample with a beam ofX-rays;

a detector assembly, which is arranged to measure a distribution of theX-rays that are emitted from the sample responsively to the beam,thereby generating an X-ray spectrum; and

a signal processor, which is adapted to assess an effect on the spectrumof a non-uniformity of the beam and to correct the spectrum responsivelyto the effect.

In some embodiments, the apparatus includes a beam-limiting optic, whichis arranged to be introduced into the beam, wherein the signal processoris adapted to assess the variation in the location due to introductionof the beam-limiting optic. The apparatus may also include a positioningassembly, which is adapted to adjust a position of the beam-limitingoptic. The detector assembly may include a detector array, whichincludes a plurality of detector elements.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for inspection of a sample, including:

an X-ray source, which is adapted to irradiate a surface of the samplewith a beam of X-rays;

a detector array, which has an axis and includes a plurality of detectorelements arranged along the axis, and which is arranged to measure adistribution of the X-rays that are emitted from the sample responsivelyto the beam; and

an alignment mechanism, which is coupled to rotate the detector array soas to position the axis perpendicular to the surface of the sample.

There is further provided, in accordance with an embodiment of thepresent invention, apparatus for inspection of a sample, including:

an X-ray source, which is adapted to irradiate a sample with a beam ofX-rays;

a detector, which is adapted to receive the X-rays reflected from alocation on the sample so as to acquire an X-ray reflectance (XRR)spectrum of the sample at the location; and

a signal processor, which is adapted to receive a tilt map, indicating acharacteristic tilt angle of a reference surface at multiple points onthe reference surface, and to determine an estimated tilt angle of thefirst location on the sample based on the tilt map, and to correct theXRR spectrum responsively to the estimated tilt angle.

There is moreover provided, in accordance with an embodiment of thepresent invention, apparatus for inspection of a sample, including:

an X-ray source, which is adapted to focus a beam of X-rays onto a focallocation on the sample;

a knife edge;

a positioning assembly, which is adapted to position the knife edge soas to intercept the beam in a position adjacent to the focal location;

a detector assembly, which is arranged to measure a distribution of theX-rays that are emitted from the sample, thereby generating an X-rayspectrum; and

a signal processor, which is adapted to receive the X-ray spectrum, andto assess an effect of the position of the knife edge on the X-rayspectrum, and to cause the positioning assembly to adjust the positionof the knife edge responsively to the effect.

There is furthermore provided, in accordance with an embodiment of thepresent invention, apparatus for inspection of a sample, including:

an X-ray source, which is adapted to irradiate the sample with a beam ofX-rays over a range of angles of incidence;

a shutter, which is positioned so as to intercept the beam at apredetermined angle;

a detector assembly, which is arranged to measure a distribution of theX-rays that are emitted from the sample responsively to the beam,thereby generating an X-ray spectrum, which includes a shadow of theshutter; and

a signal processor, which is adapted to determine a tilt angle of thesample responsively to an angular position of the shadow in thespectrum, and to process the spectrum responsively to the tilt angle.

There is also provided, in accordance with an embodiment of the presentinvention, apparatus for inspection of a sample, including:

an X-ray source, which is adapted to focus a beam of X-rays onto a focallocation on the sample;

a detector assembly, which is adapted to measure a distribution of theX-rays that are reflected from the sample responsively to the beam,thereby generating an actual reflectance spectrum; and

a signal processor, which is adapted to estimate a spot size of the beamon the sample at the focal location, to compute a simulated reflectancespectrum of the sample responsively to the spot size, and to fit thesimulated reflectance spectrum to the actual reflectance spectrum inorder to determine one or more properties of the sample.

There is additionally provided, in accordance with an embodiment of thepresent invention, a cluster tool for producing microelectronic devices,including:

a deposition station, which is adapted to form a thin-film layer on asurface of a semiconductor wafer; and

an inspection station, including:

an X-ray source, which is adapted to irradiate the semiconductor waferwith a beam of X-rays;

a detector assembly, which is arranged to measure a distribution of theX-rays that are emitted from the semiconductor wafer responsively to thebeam, thereby generating an X-ray spectrum; and

a signal processor, which is adapted to assess an effect on the spectrumof a non-uniformity of the beam and to correct the spectrum responsivelyto the effect.

There is further provided, in accordance with an embodiment of thepresent invention, apparatus for producing microelectronic devices,including:

a production chamber, which is adapted to receive a semiconductor wafer;

a deposition device, which is adapted to deposit a thin-film layer on asurface of the semiconductor wafer within the chamber;

an X-ray source, which is adapted to irradiate the semiconductor waferin the production chamber with a beam of X-rays;

a detector assembly, which is arranged to measure a distribution of theX-rays that are emitted from the semiconductor wafer responsively to thebeam, thereby generating an X-ray spectrum; and

a signal processor, which is adapted to assess an effect on the spectrumof a non-uniformity of the beam and to correct the spectrum responsivelyto the effect.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a system for XRR, in accordance withan embodiment of the present invention;

FIGS. 2A and 2B are schematic side views of a focused X-ray beam cut bya movable knife edge, in accordance with an embodiment of the presentinvention;

FIG. 3 is a schematic plot of X-ray intensity as a function of angle andof a correction factor derived from the X-ray intensity, in accordancewith an embodiment of the present invention;

FIG. 4A is a flow chart that schematically illustrates a method forcalibration of beam non-uniformities, in accordance with an embodimentof the present invention;

FIG. 4B is a schematic plot of an X-ray reflectance spectrum, inaccordance with an embodiment of the present invention;

FIG. 5 is a schematic side view of a focused X-ray beam cut by a movableknife edge and reflected from a test surface, in accordance with anembodiment of the present invention;

FIG. 6A is a schematic frontal view of a movable knife edge that is usedto cut an X-ray beam, in accordance with an embodiment of the presentinvention;

FIG. 6B is a schematic plot of X-ray intensity measured by a detectorarray as a function of the position of a knife edge that is used to cutthe X-ray beam, in accordance with an embodiment of the presentinvention;

FIG. 7 is a schematic frontal view of a sample and a detector array forreceiving X-rays reflected from the sample, illustrating a method foralignment of the detector array, in accordance with an embodiment of thepresent invention;

FIG. 8 is a schematic plot of X-ray intensity measured by a detectorarray in two different orientations, in accordance with an embodiment ofthe present invention;

FIG. 9 is a schematic representation of a tilt map, in accordance withan embodiment of the present invention;

FIG. 10A is a schematic side view of a system for XRR, showing elementsof the system that are used in measuring the tilt angle of a sample, inaccordance with an embodiment of the present invention;

FIG. 10B is a schematic plot of the intensity of X-rays reflected from asample as a function of elevation angle for two different tilt angles ofthe sample, in accordance with an embodiment of the present invention;

FIG. 11 is a schematic top view of a cluster tool for semiconductordevice fabrication, including an inspection station in accordance withan embodiment of the present invention; and

FIG. 12 is a schematic side view of a semiconductor processing chamberwith X-ray inspection capability, in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is now made to FIG. 1, which is a schematic side view of asystem 20 for X-ray reflectometry (XRR) of a sample 22, in accordancewith an embodiment of the present invention. Sample 22 is mounted on amounting assembly, such as a motion stage 24, allowing accurateadjustment of the position and orientation of the sample. An X-raysource 26 irradiates a small area 28 on sample 22 with a converging beam30 of X-rays. Typically, source 26 comprises an X-ray tube 32 withmonochromatizing optics 34. A number of different types ofmonochromatizing optics that may be used in system 20 are described inU.S. Pat. No. 6,381,303, whose disclosure is incorporated herein byreference. For example, the optics may comprise a curved crystalmonochromator, such as the Doubly-Bent Focusing Crystal Optic, producedby XOS Inc., of Albany, N.Y. Other suitable optics are described in theabove-mentioned U.S. Pat. No. 5,619,548. The doubly-curved focusingcrystal causes beam 30 to converge in both the horizontal and verticaldirections. Alternatively, a cylindrical optic may be used to focus beam30 so that the beam converges to a line on the sample surface. Furtherpossible optical configurations will be apparent to those skilled in theart.

X-rays are reflected from sample 22 in a diverging beam 38. Thereflected X-rays are collected by a detector assembly 40, whichcomprises a detector array 42, such as a CCD array. Although forsimplicity of illustration, only a single row of detectors elements isshown in the figures, with a relatively small number of detectorelements, array 42 generally includes a greater number of elements,arranged as either a linear array or a matrix (two-dimensional) array.Detector assembly further comprises an alignment mechanism 44, foradjusting the position and orientation of array 42 relative to sample 22and to the other components of system 20. Further aspects of detectorassembly 40 are described in the above-mentioned U.S. Pat. No.6,512,814.

A signal processor 46 receives and analyzes the output of assembly 40,so as to determine a distribution 48 of the flux of X-ray photonsreflected from sample 22 as a function of angle at a given energy orover a range of energies. Typically, sample 22 has one or more thinsurface layers, such as thin films, at area 28. Consequently,distribution 48 as a function of elevation angle exhibits a structurethat is characteristic of interference and/or diffraction effects due tothe surface layer and interfaces between the layers. Processor 46analyzes the angular distribution in order to determine characteristicsof one or more of the surface layers of the sample, such as thethickness, density, composition and surface quality of the layer, usingmethods of analysis known in the art. Such methods are described, forexample, in the above-mentioned U.S. Pat. No. 6,512,814.

It can be seen in FIG. 1 that because of the low range of angles atwhich beam 30 is incident on sample 22, typically below 5°, the focalspot of the beam is elongated laterally along the beam axis (i.e., alongthe Y-axis in FIG. 1). The elongation is particularly pronounced at verylow angles, near 0°. Therefore, to enhance the resolution of low-anglemeasurements, a knife edge 36 is positioned to cut the upper portion ofthe incident beam. For measurements at low incidence angles, the knifeedge is lowered very near to the surface, intercepting the incidentX-ray beam and thus shortening the lateral dimension of the spot on thesurface. For high-angle measurements, the knife edge may be raised outof the way, to allow the full intensity of the X-ray beam to be used.Such operation of the knife edge allows measurements to be made withhigh spatial resolution, particularly at low angles at which the lateraldimension of the spot is most greatly elongated, while maintaining highsensitivity even at high angles. Knife edge 36 may be used inconjunction with a dynamic shutter (not shown), as described in theabove-mentioned U.S. Pat. No. 6,512,814.

When knife edge 36 is lowered sufficiently, most of incident beam 30 iscut off, and the lateral dimension of the X-ray spot on area 28 isreduced. Typically, the knife edge is lowered to within less than 10 μmof the surface of sample 22, and possibly to as little as 1 μm or lessfrom the surface. The lateral dimension of the spot may thus be reducedto 0.5 mm or less, instead of the typical dimension of 5 mm or more whenthe knife-edge is not used. A vertical slit (not shown) may also be usedto reduce the transverse dimension of the spot on the sample surface.The reduced spot size on the sample means that low-angle reflectionmeasurements made by system 20 have enhanced spatial resolution,providing more detailed information about thin film microstructures onsample 22. Alternatively or additionally, when a certain area of thesample, such as a patterned semiconductor wafer, must be set aside fortesting, the small spot size enables a smaller portion of sample “realestate” to be used for this purpose.

Ideally, if incident beam 30 were perfectly uniform, insertion of knifeedge 36 into the beam would reduce the intensity of reflected beam 38(as well as of the direct beam, which strikes array 42 withoutreflection) uniformly as a function of angle, but would not change therelative angular distribution of the reflected radiation. In practice,however, the incident beam is not uniform, due, for example, toaberrations in optics 34. Therefore, the shape of distribution 48typically changes depending on the position of knife edge 36. Thesechanges can compromise the accuracy of the thin film measurements thatare derived from the XRR spectra. Embodiments of the present inventionthat are described hereinbelow provide methods for measuring andcompensating for the effect of knife edge 36 on the non-uniform beam.

FIGS. 2A and 2B are schematic side views showing beam 30 cut by knifeedge 36 in two different positions 54 and 56 during a calibrationprocedure, in accordance with an embodiment of the present invention.For the purpose of this procedure, sample 22 and stage 24 are removedfrom the area of beam 30, so that beam 38 strikes detector array 42directly, without reflection from any intervening surface. Knife edge 36intercepts beam 30 at a beam waist 50, corresponding to the position ofthe focus of beam 30 in area 28 on the surface of sample 22 (FIG. 1). Inposition 54, the lower tip of the knife edge is located at a certain,small distance (for example, 10 μm) above the plane of the samplesurface, whereas in position 56, the lower tip of the knife edge islocated at the same distance (10 μm) below the surface plane. Signalsreceived by elements 52 of detector array 42 in both positions of theknife edge are conveyed to processor 46 for analysis.

Reference is now made to FIGS. 3 and 4A, which schematically illustratea procedure for calibration of non-uniformity effects due to knife edge36, in accordance with an embodiment of the present invention. FIG. 3 isa plot that schematically illustrates simulated results of themeasurements made in the configurations of FIGS. 2A and 2B, while FIG.4A is a flow chart showing the steps in the calibration procedure. Theprocedure starts with a measurement of X-ray flux by array 42 whileknife edge 36 is in its normal position for XRR measurement, such as inposition 54 (FIG. 2A), at an actual measurement step 70. A trace 60 inFIG. 3 shows the X-ray flux measured by array 42 with knife edge 36 inthis position 54. The results are shown in units of electrons countedper element 52 as a function of elevation angle. The measurement isrepeated with the knife edge in a reference position, such as position56 (FIG. 2B), at a reference measurement step 72. A trace 62 in FIG. 3shows this reference measurement.

The measurements made at steps 70 and 72 are used in determining acalibration vector, to compensate for the non-uniformity of beam 30, ata calibration step 74. For this purpose, processor 46 calculates thedifference between traces 60 and 62 as a function of angle. Thisdifference is illustrated by a trace 64 in FIG. 3. The processor thencalculates a correction vector based on the difference indicated bytrace 64. The correction vector comprises a scalar correction factor asa function of angle, as shown by a trace 66 in FIG. 3. In the presentexample, the correction factor is determined by taking the inverse ofthe difference between traced 60 and 62, and then normalizing so thatapplication of the correction vector does not change the averageintensity of the spectrum. Steps 70-74 may be repeated for multipledifferent heights of knife edge 36 in order to generate a set ofcorrection vectors for the different height settings. Correction vectorsfor intermediate height settings are determined by interpolation.

Sample 22 is now introduced and aligned in the path of beam 30, withknife edge 36 in position 54, at an XRR measurement step 75. Inpreparation for or as part of the measurement, the tilt angle andvertical (Z) position of area 28 are determined relative to a known zeroreference. Any suitable method may be used for determining the tiltangle and vertical position, such as the methods described hereinbelowor methods described in U.S. patent application Ser. No. 10/689,314,filed Oct. 20, 2003, which is assigned to the assignee of the presentpatent application, and whose disclosure is incorporated herein byreference, or other methods known in the art. XRR distribution 48 ismeasured by array 42 as a function of elevation angle. The angular scaleof the measurement is corrected for the tilt angle and vertical positionof area 28. The measured XRR output of each element 52 of detector array42 is then multiplied by the corresponding correction factor for therespective angle of reflection measured by the element, at a correctionstep 76. (Alternatively, the correction vector may be adjusted beforemultiplication by the tilt angle and vertical position of area 28.) As aresult, the XRR spectrum more accurately reflects the actual structureof the surface layers of the sample, while artifacts due to beamnon-uniformity are reduced.

FIG. 4B is a schematic plot showing an exemplary XRR spectrum 77 of asample, in accordance with an embodiment of the present invention. Thespectrum shown in FIG. 4B is taken from the above-mentioned U.S. Pat.No. 6,512,814, and details of the steps used in capturing this spectrumare described there. The intensity of the individual signals generatedby elements 52 of array 42 are corrected as described above at step 76.The resulting spectrum 77 contains a fringe pattern of peaks 78 andintervening troughs 79 whose characteristics are indicative of thesurface structure of sample 22. Further aspects of the analysis of thisfringe pattern are described hereinbelow.

FIG. 5 shows another method for calibrating the non-uniformity ofincident beam 30, in accordance with an alternative embodiment of thepresent invention. For the purposes of this method, a reflectivereference sample 80 is placed on stage 24 in place of the actual testsample. Typically, sample 80 comprises A polished metal surface. Array42 measures X-ray reflection from the surface of sample 80 with knifeedge 36 in actual measurement position 54 at step 70 (FIG. 4A). Themeasurement is repeated with knife edge 36 in a reference position awayfrom the surface of the sample, so that the knife edge does notintercept beam 30, at step 72. To determine the calibration vector atstep 74, processor 46 divides the X-ray flux measured by each element 52with knife edge 36 in the reference position by the flux measured withthe knife edge in position 54. This calibration vector is then appliedto actual XRR spectra of sample 22 at step 76.

The use of knife edge 36 to intercept beam 30 may also shift thelocation of the focal point of beam 30 in area 28, relative to the focalpoint when the knife edge is removed from the beam. In this context,since the focal spot typically extends over a length of one to severalmillimeters on the surface of sample 22, the focal point is defined bythe location of the center of mass of the distribution of X-rayradiation on the surface. The respective elevation angle φ (FIG. 1) thatcorresponds to each element 52 of detector array 42 is given byarctan(h/d), wherein h is the height of the element above the sampleplane, and d is the distance from the focal point to the detector array.Accurate analysis of the XRR spectra requires that the elevation anglesbe known very precisely. Movement of the focal point of beam 30 due tothe use of knife edge 36 can change the distance between the focal pointand the detector array, and may thus compromise the accuracy of XRRmeasurements.

In order to overcome this problem, the shift in focal point location ismeasured as a function of the position of knife edge 36. One method thatcan be used to measure the focal point location is to place a referencesample of known characteristics on stage 24, in place of sample 80 inFIG. 5, and measure the XRR spectrum of the sample. If the referencesample has a thin film surface layer of known thickness, then the XRRspectrum will comprise a pattern of fringes with known angular spacing,as shown above in FIG. 4B. Since the angular spacing of the fringes isconstant, the fringe spacing in spectrum 77 is expected to be identicalfor all positions of knife edge 36 as long as there is no variation inthe focal point location. In practice, the focal point location shiftswith knife edge position due to non-uniformity of beam 30, and thefringe spacing changes accordingly due to the variation in the distancebetween the focal point and detector array 42.

Thus, the dependence of focal point location on the knife edge positionmay be determined by measuring the changes in fringe spacing fordifferent positions of the knife edge. Alignment mechanism 44 indetector assembly 40 is then used to adjust the lateral position ofarray 42 (i.e., to shift the entire array forward or back along theY-direction) based on this measurement, so that the distance from thefocal point to the array remains constant regardless of the position ofknife edge 36. Alternatively, array 42 may be held stationary, and themapping of detector elements 52 to elevation angle φ may be adjusted toaccount for the varying distance d. The measurement and calculation ofadjustment factors may be performed in advance for different settings ofthe height of knife edge 36, and then applied in actual measurementsdepending on the actual position of the knife edge.

Alternatively or additionally, the lateral (Y-direction) location ofknife edge 36 may be adjusted in order to minimize the variation infocal point location with the height (Z-axis position) of the knifeedge. If converging beam 30 were ideally focused to waist 50 (FIG. 2A),and knife edge 36 were position precisely at the waist, then the focalpoint location of the beam would not change with variation of the heightof the knife edge. Displacement of the knife edge in the Y-directionfrom the waist location, however, causes the focal point location toshift with knife height. In order to minimize this variation, thevariation in focal point location with knife height may be measured at anumber of different positions of the knife edge along the Y-direction.The Y-position of the knife edge is then set at the location thatminimizes the focal point shift.

Alternatively or additionally, other methods may be used to measure andcorrect for beam non-uniformities introduced by the use of knife edge36. All such methods are considered to be within the scope of thepresent invention. For example, it is also possible to measure thelocation of the center of mass of the X-ray focal spot in area 28separately for different reflection angles (and hence for differentelements 52 of array 42). In one embodiment, this measurement isperformed by observing the relative variation in the X-ray intensitymeasured by each element 52 as a function of variations in the positionof knife edge 36. It is thus possible to find and correct for thevariation in focal point position as a function of the angle, as well asof the height of the knife edge. The measured variations in X-rayintensity as a function of knife edge position may also be used todetermine the effective focal spot size as a function of reflectionangle. In this case, a non-linear correction may be applied to themapping of detector elements 52 to elevation angle φ.

The focal spot size of X-ray beam in area 28 is also significant in theangular resolution of the spectrum. If the beam could be focused to aprecise point, each element 52 of array 42 would receive X-raysreflected from sample 22 in its own, unique angular range, determinedonly by the height of the detector element and its distance from thefocal spot. Because the focal spot spreads over a certain area, however,each element 52 receives X-rays over a larger angular range, whichpartially overlaps the angular ranges of the neighboring elements. As aresult, a certain amount of blur is introduced into spectrum 77 (FIG.4B). For example, the blur can be estimated as follows:Blur[deg]=(Effective vertical spot size+Detector elementheight)/(detector pitch)*Angle step;Here the angle step is the angular distance (in degrees) between twoadjacent elements 52, and the effective vertical spot size is equal totwice the height of knife edge 36 above the surface of sample 22.

Alternatively, other measures of the spot size and the resulting blurmay be used. For example, instead of assuming a constant effectivevertical spot size in the above formula, a variable measure of the spotsize as a function of reflection angle may be used, giving a resultingvariation in the estimated blur at different reflection angles. Methodsfor measuring the effective spot size as a function of angle aredescribed above.

The above-mentioned U.S. Pat. No. 6,512,814 describes a method forparametric modeling of XRR spectra in order to determine properties ofthin film layers on sample 22 (such as the layer thickness, density andsurface roughness). These properties are determined by fitting simulatedspectra calculated by the parametric model to the actual spectracaptured by system 20. In an embodiment of the present invention, inorder to optimize the accuracy of the fit, the simulated spectrum isblurred by convolution with a blur function, and it is the blurredspectrum that is fitted to the actual spectrum in order to determine thethin film properties. The extent of the convolution kernel is given bythe height of knife edge 36, in accordance with the above formula.

Reference is now made to FIGS. 6A and 6B, which schematically illustratea method for aligning knife edge 36, in accordance with an embodiment ofthe present invention. FIG. 6A is a schematic frontal view of knife edge36 together with a positioning assembly 81 that is used in controllingthe position and orientation of the knife edge. Typically, signalprocessor 46 (FIG. 1) controls assembly 81 based on measurements made bydetector assembly 40, as described hereinbelow. The skew angle β of theknife edge is exaggerated in the figure for visual clarity. FIG. 6B is aplot of measurements of X-ray intensity measured in system 20 as afunction of the height (Z-axis position) of the knife edge. Themeasurements are presented at three different settings of the skew angleβ, giving three different curves 82, 83 and 84.

If knife edge 36 were precisely parallel to the surface of sample 22,then the X-ray intensity would drop to zero at zero height. In practice,there is almost always a small amount of tilt or other residualimperfection that permits some radiation to pass between the knife edgeand the sample surface. The skew angle of the knife edge is adjusted,using positioning assembly 81, in order to minimize the amount ofradiation passing through to the detector array at zero height. In thisexample, the skew would be set to the value that generated curve 84.Furthermore, curves 82, 83 and/or 84 may be extrapolated down to zerointensity in order to calibrate the zero point of the knife heightscale. Assembly 81 may also be used in adjusting the height and theY-displacement of knife edge 36, as described above.

FIG. 7 is a schematic frontal view of detector array 42, illustratinganother aspect of the calibration of system 20, in accordance with anembodiment of the present invention. Ideally, for optimal angularresolution and accuracy, array 42 is aligned so that an axis 84 of thearray is perpendicular to the surface of sample 22. In practice,however, the sample surface may tilt, or the array itself may becomemisaligned. The result of this sort of tilt or other misalignment isshown in the form of a “shadow array” 86 in FIG. 7. The tilt of array 86is exaggerated for clarity of illustration.

FIG. 8 is a schematic plot of X-ray intensity as a function of angle φ,which illustrates a method for aligning array 42 relative to sample 22,in accordance with an embodiment of the present invention. The angle ismeasured along axis 84 of array 42. To generate plots of this sort, thesample is irradiated with an incident beam at a grazing angle, i.e.,roughly parallel to the surface of the sample. As a result, the X-rayflux measured by elements 52 of array 42 that are above the plane of thesurface is high, while that measured by the elements below the plane islow. This result is shown in FIG. 8, wherein the dashed vertical line inthe figure corresponds to the location of the surface plane of sample22. The X-ray intensity measured by elements 52 above the surface planeappears to the right of this vertical line, while the intensity measuredby the elements below the surface plane appears to the left.

A trace 90 in FIG. 8 corresponds to the intensity measured by “array”86, while a trace 92 corresponds to the intensity measured when array 42is properly aligned, with axis 84 perpendicular to the sample surfaceplane. Trace 92 is characterized by a sharp transition from low to highintensity at the surface plane. In trace 90, however, the transition ismore gradual, since elements 52 near the surface plane are partlyexposed to the incident X-rays and partly obscured by sample 22. Inorder to align array 42 so that axis 84 is precisely perpendicular tothe sample surface plane, alignment mechanism 44 is operated to rotatethe array while processor 46 monitors the sharpness of the transitionfrom low to high flux in the intensity trace of FIG. 8. The angularorientation of array 42 is fixed at the angle that gives the sharpesttransition.

Alternatively, other criteria may be used to determine the optimalorientation of axis 84 of array 42. For example, when axis 84 isproperly aligned perpendicular to the surface of sample 22, the fringepattern in the XRR spectrum of the sample will have the greatestcontrast, i.e., the difference between the intensities of the peaks ofthe fringes relative to the troughs between the fringes is maximized. Asaxis 84 is tilted, this contrast gradually decreases, for the samereason as the transition in trace 92 is sharper than that in trace 90.Therefore, array 42 may be aligned by observing the XRR fringe patternwhile rotating axis 84, and choosing the rotation angle that gives thefringes of highest contrast.

FIG. 9 is a schematic representation of a tilt map 100, in accordancewith an embodiment of the present invention. As noted earlier, stage 24shifts sample 22 in the X-Y plane to enable system 20 to measure XRRspectra at multiple locations on the surface of the sample. The surfacetilt angle of the sample (i.e., the angle of deviation between a planethat is locally tangent to the surface and the reference X-Y plane) onstage 24 may not be perfectly uniform over the entire surface of thesample. For example, in a typical application of system 20, sample 22 isa semiconductor wafer, which is held in place on stage 24 by suctionexerted through vacuum ports (not shown) in the surface of the stage.Under these circumstances, the wafer conforms to the shape of the stage,with deformations due to the force of the suction. As a result, thelocal tilt angle of the wafer may vary from point to point on the wafersurface. Accurate XRR measurement, however, requires that the tilt angleat each point be known and taken into account.

Tilt map 100 is generated and used in order to facilitate compensationfor these tilt angle variations. To produce the map, a reference sampleis loaded onto stage 24. The surface of the sample is divided intoregions 102, and the surface tilt is measured in each region to give atilt value 104. The tilt value is typically expressed in terms of tiltangles of the surface about the X- and Y-axes, identified in the figureas 74 _(X) and θ_(Y). Any suitable method that is known in the art maybe used to determine the surface tilt. For example, the zero-angle ofthe surface may be determined in system 20 by means of X-ray measurementtechniques, such as the techniques described in the above-mentioned U.S.patent applications Ser. Nos. 10/313,280, 10/364,883, and 10/689,314, orin U.S. Pat. No. 6,680,996, which is also incorporated herein byreference. Another method for measuring tilt using X-rays is describedhereinbelow with reference to FIGS. 10A and 10B. Alternatively, opticaltechniques may be used to determine the surface tilt, such as thetechnique described in the above-mentioned U.S. Patent ApplicationPublication US 2004/0052330 A1.

Generating tilt map 100 with a dense grid of regions 102 istime-consuming, but it need be performed only once, typically bymeasuring the tilt over the surfaces of a number of reference samples,and then averaging the results. Subsequently, the tilt measurements maybe repeated from time to time (at some or all of regions 102 in the tiltmap), and the tilt map may be updated accordingly. The tilt angles ofother locations on sample 22 are determined by interpolation, based onthe measurements and on the tilt characteristics given by map 100. Forexample, a spline curve or curved surface may be fitted to map 100, andthe tilt angle at any desired location on sample 22 may then be found byinterpolating along the curve or surface, using the small number ofpoints at which the tilt was actually measured as reference points.

In some embodiments, stage 24 is capable of both X-Y translationalmovement and rotation about the Z-axis. (The rotation angle is referredto hereinbelow as θ_(Z).) Thus, a complete tilt map for a system usingsuch a stage will take into account not only the X-Y grid shown in FIG.9, but also the rotation angle. The most precise method for X-Y-θ_(Z)mapping is to map the entire X-Y grid at multiple different rotationangles. Alternatively, a less time-consuming option would be to make asingle two-dimensional map (such as an X-Y map), and then make a furthermeasurement of the effect of movement in the third dimension (forexample, measuring the tilt as a function of θ_(Z) at one or a fewpoints in the X-Y plane). A further option would be to simply map thetilt as a separate function of movement in the X, Y and θ_(Z)directions. Furthermore, the tilt measurements and interpolation may beperformed separately for tilt about each of the X- and Y-axes, oralternatively, combined X/Y tilt values may be used. Other methods ofmeasurement and interpolation will be apparent to those skilled in theart.

The interpolated tilt values are then applied in order to correct theXRR results. Typically, the angular scale of each distribution 48 isadjusted to account for the local tilt at the point at which thedistribution was measured. Alternatively, the tilt angle of stage 24 orthe positions of X-ray source 26 and detector array 42 may be adjustedto compensate for the local tilt. Similarly, the position andorientation of knife edge 36 may be adjusted based on the tilt map sothat the knife edge is parallel to the surface of sample 22 at theproper height. When a shutter is used to cut off low-angle radiation (asshown below in FIG. 10A), the shutter angle and height may be similarlyadjusted. Other parameters that are used in analyzing the XRR spectrum,such as the beam blur, focal distance of the detector, and thecalibration vector, may also be adjusted based on the tilt map.

FIG. 10A is a schematic side view showing elements of system 20 used ina method for measuring tilt of sample 22, in accordance with anembodiment of the present invention. In this embodiment, detector array42 is used to measure the component of the tilt of area 28 in the θdirection, i.e., about the X-axis (FIG. 1). For this purpose, a shutter110 is introduced into the incident converging X-ray beam. The height ofshutter 110 is chosen so that the shutter cuts off the incidentradiation below a selected angle φ_(SHUTTER). As a result the shuttercasts a shadow on array 42 at a corresponding angle φ_(SHADOW) andbelow. (For specular reflection, in the sample tilt angle θ=0, thenφ_(SHUTTER)=φ_(SHADOW).) Typically, φ_(SHUTTER) is chosen to be lessthan the critical angle of sample 22 for total external reflection,φ_(CRIT). For example, assuming sample 22 to comprise a silicon wafer,with φ_(CRIT)=0.227° at 8.05 keV (CuKal), φ_(SHUTTER) may be set toabout 0.1°.

FIG. 10B is a schematic plot showing reflected X-ray intensity fromsample 22 as a function of elevation angle φ, as measured by array 42 inthe configuration of FIG. 10A, in accordance with an embodiment of thepresent invention. Two curves are shown in FIG. 10B:

-   -   A reference curve 112, which is measured with sample 22 held in        a reference orientation, typically with tilt θ=0; and    -   An uncorrected curve 114, which is measured with sample 22        tilted by a small angle δθ.        Curve 112 is characterized by a left shoulder at angle        φ_(SHADOW), marking the upper edge of the shadow cast by shutter        110, and by a right shoulder at angle Φ_(CRIT), above which the        XRR spectrum falls off sharply, as is known in the art. Curve        114 is similar in shape to curve 112, except that the XRR        spectrum received from the tilted sample is shifted by the        amount of the tilt angle δθ. Thus, as shown in FIG. 10B, the        right shoulder of curve 114 is shifted by δθ relative to curve        112. On the other hand, by the principles of specular        reflection, the point at which shutter 110 is reflected onto        array 42 shifts by 2δθ when sample 22 is tilted relative to the        reference position. Therefore, as shown in FIG. 10B, the left        shoulder of curve 114 is shifted by 2δθ relative to curve 112.

The characteristics of the curves shown in FIG. 10B may be used inmeasuring and correcting for the tilt angles of different samples thatare used in system 20. Although the right shoulders of curves 112 and114 are displaced by the tilt angle δθ, this relation will hold only aslong as curves 112 and 114 are generated using the same sample (or atleast samples of the same material). On the other hand, the relativedisplacement of the left shoulders is a geometrical effect, independentof the sample materials. Thus, the tilt angle θ of substantially anyflat sample may be determined, relative to the known referenceorientation at which curve 112 is recorded, using the formula:θ=θ_(REF)+½[current shadow angle−reference shadow angle]wherein the shadow angle is determined, for example, by the location ofthe left shoulder in the XRR curves in question. This approach permitsthe tilt angle of sample 22 to be determined simply and accurately foreach area on the sample that is inspected by XRR with no more thanminimal added measurement and computation.

When shutter 110 is used in system 20, it is important that the shutterbe precisely parallel to the surface of sample 22 along the X-direction.As noted above, the shutter angle may be adjusted on the basis of thetilt map, which is shown in FIG. 9. Additionally or alternatively, theshutter may be adjusted so as to maximize the sharpness of the shadow ofthe shutter, as seen in curves 112 and 114. Alternatively, if it is notpossible or desirable to adjust the shutter (as well as knife edge 36)to exactly parallel the surface of sample 22 at all positions andorientations of the sample, the effects of the known tilt of the shutter(and/or knife edge) relative to the surface may be taken into account inmodeling the XRR spectrum, as described above. These effects mayinclude, for example, blur of the X-ray beam and variations in theeffective distance between the focal spot on sample 22 and the elementsof array 42.

FIG. 11 is a schematic top view of a cluster tool 120 for use insemiconductor device fabrication, in accordance with an embodiment ofthe present invention. The cluster tool comprises multiple stations,including a deposition station 122, for depositing thin films on asemiconductor wafer 130, an inspection station 124, and other stations126, 128, as are known in the art, such as a cleaning station.Inspection station 124 is constructed and operates in a manner similarto system 20, as described hereinabove. A robot 132 transfers wafer 130among stations 122, 124, 126, . . . , under the control of a systemcontroller 134. Operation of tool 120 may be controlled and monitored byan operator using a workstation 136, coupled to controller 134.

Inspection station 124 is used to perform X-ray inspection of wafers byXRR. Such inspection is typically carried out before and/or afterselected steps in production processes carried out by deposition station122 and other stations in tool 120. The inspection station is calibratedusing the methods described above. Use of station 124 allows earlydetection of process deviations and convenient adjustment and evaluationof process parameters on production wafers, using controller 134 andpossibly workstation 136.

FIG. 12 is a schematic side view of a system 140 for semiconductor waferfabrication and in situ inspection, in accordance with anotherembodiment of the present invention. System 140 comprises a vacuumchamber 142, containing deposition apparatus 144, for creating thinfilms on wafer 130, as is known in the art. The wafer is mounted onmotion stage 24 within chamber 142. The chamber typically comprisesX-ray windows 146, which may be of the type described in theabove-mentioned Patent Application Publication US 2001/0043668 A1. X-raysource 26 irradiates area 28 on wafer 130 via one of windows 146, in themanner described above. Some of the elements shown in FIG. 1 are omittedfrom FIG. 10 for the sake of simplicity, but typically, elements of thissort are integrated into system 140, as well.

X-rays reflected or diffracted from area 28 are received by array 42 indetector assembly 40 via another one of windows 146. Processor 46receives signals from detector assembly 40, and processes the signals inorder to assess characteristics of thin-film layers in production withinchamber 142, by measuring the XRR spectrum of wafer 130. The elements ofsystem 140 that are used in the XRR measurement are calibrated in themanner described above. The results of the XRR assessment may be used incontrolling deposition apparatus 144 so that the films produced bysystem 140 have desired characteristics, such as thickness, density,composition and surface roughness.

Although the embodiments described above refer specifically to X-rayreflectometry, the principles of the present invention may similarly beused, mutatis mutandis, in other fields of X-ray analysis. Exemplaryfields of application include X-ray fluorescence (XRF) analysis,including particularly grazing emission XRF, as well as other XRFtechniques known in the art. Grazing emission XRF is described, forexample, in an article by Wiener et al., entitled “Characterization ofTitanium Nitride Layers by Grazing-Emission X-ray FluorescenceSpectrometry,” in Applied Surface Science 125 (1998), p. 129, which isincorporated herein by reference. X-ray fluorescence measurement may beincorporated in system 20, as described in the above-mentioned U.S. Pat.No. 6,381,303, for example. Additionally or alternatively, the systemmay be adapted to make small-angle scattering measurements, as describedin the above-mentioned U.S. patent application Ser. No. 10/364,883, orX-ray diffraction measurements. Furthermore, the principles of system 20may be implemented in position-sensitive detection systems for otherenergy ranges, such as for detection of gamma rays and other nuclearradiation.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

1. A method for inspection of a sample, comprising: irradiating thesample with a beam of X-rays; measuring a distribution of the X-raysthat are emitted from the sample responsively to the beam, therebygenerating an X-ray spectrum; assessing an effect on the spectrum of anon-uniformity of the beam; and correcting the spectrum responsively tothe effect.
 2. The method according to claim 1, wherein measuring thedistribution comprises measuring the distribution of the X-rays that arereflected from the sample as a function of an elevation angle relativeto a surface of the sample.
 3. The method according to claim 2, whereinirradiating the sample comprises focusing the beam so that the X-raysconverge on the sample over a range of incidence angles.
 4. The methodaccording to claim 3, wherein focusing the beam comprises forming afocal spot on the surface of the sample, and wherein assessing theeffect comprises assessing a variation in a location of the focal spot.5. The method according to claim 4, wherein measuring the distributionof the X-rays comprises receiving the reflected X-rays at a detector,and wherein correcting the spectrum comprises adjusting a position ofthe detector responsively to the variation in the location of the focalspot.
 6. The method according to claim 5, wherein adjusting the positioncomprises aligning the detector so as to maintain a constant distancebetween the focal spot and the detector notwithstanding the variation inthe location of the focal spot.
 7. The method according to claim 4,wherein irradiating the sample comprises introducing a beam-limitingoptic into the beam, and wherein assessing the effect comprises findingthe variation in the location due to introduction of the beam-limitingoptic.
 8. The method according to claim 7, wherein introducing thebeam-limiting optic comprises positioning a knife edge so as tointercept the beam in a position adjacent to the focal spot.
 9. Themethod according to claim 8, wherein positioning the knife edgecomprises adjusting a position of the knife edge, and wherein findingthe variation in the location comprises measuring the variation as afunction of the position of the knife edge.
 10. The method according toclaim 9, wherein adjusting the position comprises adjusting a height ofthe knife edge relative to the surface of the sample, and whereincorrecting the spectrum comprises determining a correction to apply tothe spectrum responsively to the height.
 11. The method according toclaim 9, wherein adjusting the position comprises adjusting a lateralposition of the knife edge relative to the beam, and wherein correctingthe spectrum comprises selecting the lateral position so as to minimizethe effect of the non-uniformity.
 12. The method according to claim 4,wherein measuring the distribution of the X-rays comprises recordingmeasurement values as a function of the elevation angle, and whereincorrecting the spectrum comprises modifying the measurement values toaccount for the variation in the location of the focal spot.
 13. Themethod according to claim 12, wherein assessing the variation comprisesdetermining an effective variation in the location of the focal spot asa function of the elevation angle, and wherein modifying the measurementvalues comprises adjusting a mapping of the measurement values toelevation angles responsively to the effective variation.
 14. The methodaccording to claim 2, wherein irradiating the sample comprisesintroducing a beam-limiting optic into the beam, and wherein assessingthe effect comprises measuring a variation in the beam as a function ofthe elevation angle due to introduction of the beam-limiting optic. 15.The method according to claim 14, wherein irradiating the samplecomprises directing the beam to impinge on the sample at a focallocation, and wherein introducing the beam-limiting optic comprisespositioning a knife edge so as to intercept the beam in a positionadjacent to the focal location.
 16. The method according to claim 15,wherein positioning the knife edge comprises adjusting a height of theknife edge relative to the surface of the sample, and wherein correctingthe spectrum comprises determining a correction to apply to the spectrumresponsively to the height.
 17. The method according to claim 15,wherein positioning the knife edge comprises adjusting a lateralposition of the knife edge relative to the beam, and wherein correctingthe spectrum comprises selecting the lateral position so as to minimizethe effect of the non-uniformity.
 18. The method according to claim 14,wherein assessing the effect comprises determining a correction factoras a function of the elevation angle responsively to the measuredvariation, and wherein correcting the spectrum comprises applying thecorrection factor to the spectrum.
 19. The method according to claim 18,wherein determining the correction factor comprises: directing the beamtoward a detector array comprising a plurality of detector elements;making a first measurement of a flux of the X-rays that is incident oneach of the detector elements with the beam-limiting optic in a firstposition; making a second measurement of the flux of the X-rays that isincident on each of the detector elements with the beam-limiting opticin a second position; and comparing the first and second measurements inorder to determine the correction factor.
 20. The method according toclaim 19, wherein measuring the distribution of the X-rays that arereflected from the sample comprises measuring the distribution using thedetector array with the beam-limiting optic in the first position. 21.The method according to claim 20, wherein making the first and secondmeasurements comprises removing the sample from the beam of X-rays sothat the beam is directly incident on the detector array.
 22. The methodaccording to claim 20, wherein making the first and second measurementscomprises introducing a reflective surface into the beam at a locationof the sample so that the X-rays are reflected onto the detector array.23. The method according to claim 1, wherein irradiating the samplecomprises introducing a beam-limiting optic into the beam, and whereinassessing the effect comprises measuring a variation in the beam due tointroduction of the beam-limiting optic.
 24. The method according toclaim 23, wherein assessing the effect comprises determining acorrection vector responsively to the measured variation, and whereincorrecting the spectrum comprises applying the correction vector to thespectrum.
 25. The method according to claim 24, wherein determining thecorrection vector comprises: directing the beam toward a detector arraycomprising a plurality of detector elements; making a first measurementof a flux of the X-rays that is incident on each of the detectorelements with the beam-limiting optic in a first position; making asecond measurement of the flux of the X-rays that is incident on each ofthe detector elements with the beam-limiting optic in a second position;and comparing the first and second measurements in order to determinethe correction vector.
 26. The method according to claim 25, wherein thedetector array has an axis, and wherein the method comprises rotatingthe array so as to position the axis perpendicular to a surface of thesample.
 27. The method according to claim 1, wherein measuring thedistribution comprises: providing a tilt map indicating a characteristictilt angle of the sample at multiple points over a surface of thesample; determining, based on the tilt map, a local tilt angle at alocation on the sample upon which the beam of X-rays is incident; andcorrecting the spectrum responsively to the local tilt angle.
 28. Amethod for inspection of a sample, comprising: irradiating a surface ofthe sample with a beam of X-rays; measuring a distribution of the X-raysthat are emitted from the sample responsively to the beam using adetector array, which has an axis and comprises a plurality of detectorelements arranged along the axis; and rotating the detector array so asto position the axis perpendicular to the surface of the sample.
 29. Themethod according to claim 28, wherein measuring the distributioncomprises measuring the distribution of the X-rays that are reflectedfrom the sample as a function of an elevation angle relative to asurface of the sample.
 30. The method according to claim 29, whereinmeasuring the distribution of the X-rays comprises observing anoscillatory pattern in the X-rays emitted as a function of the elevationangle, and rotating the detector array comprises aligning the detectorarray responsively to the oscillatory pattern.
 31. The method accordingto claim 30, wherein aligning the detector array comprises rotating thedetector array so as to maximize a contrast of the oscillatory pattern.32. The method according to claim 28, wherein irradiating the surfacecomprises directing the beam toward the surface at a grazing incidence,and wherein measuring the distribution comprises detecting a transitionin the distribution corresponding to a plane of the surface of thesample, and wherein rotating the detector array comprises aligning thedetector array responsively to the transition.
 33. The method accordingto claim 30, wherein aligning the detector array comprises rotating thedetector array so as to maximize a sharpness of the transition.
 34. Amethod for inspection of a sample, comprising: providing a tilt map,indicating a characteristic tilt angle of a reference surface atmultiple points on the reference surface; acquiring an X-ray reflectance(XRR) spectrum at a location on a sample; determining an estimated tiltangle of the location on the sample based on the tilt map; andcorrecting the XRR spectrum responsively to the estimated tilt angle.35. The method according to claim 34, wherein providing the tilt mapcomprises measuring the characteristic tilt angle of the referencesurface at each of the multiple points.
 36. The method according toclaim 35, wherein acquiring the XRR spectrum comprises mounting thesample on a mounting assembly, and wherein measuring the characteristictilt angle comprises mounting the reference surface on the mountingassembly, and measuring the characteristic tilt angle while thereference surface is on the mounting assembly.
 37. The method accordingto claim 36, wherein measuring the characteristic tilt angle comprisesrotating and translating the mounting assembly, and taking measurementsof the tilt angle as a function of rotation and translation.
 38. Themethod according to claim 34, wherein determining the estimated tiltangle comprises finding the estimated tilt angle by interpolation alonga curve in the tilt map.
 39. The method according to claim 34, whereinthe sample comprises a semiconductor wafer.
 40. A method for inspectionof a sample, comprising: focusing a beam of X-rays onto a focal locationon the sample; positioning a knife edge so as to intercept the beam in aposition adjacent to the focal location; measuring a distribution of theX-rays that are emitted from the sample responsively to the beam and tothe position of the knife edge; and adjusting the position of the knifeedge responsively to the distribution.
 41. The method according to claim40, wherein adjusting the position comprises adjusting a laterallocation of the knife edge relative to the beam.
 42. The methodaccording to claim 41, wherein measuring the distribution comprisesmeasuring a variation in the distribution of the X-rays as a function ofa vertical location of the knife edge relative to a surface of thesample at each of a plurality of lateral locations of the knife edge,and wherein adjusting the lateral location comprises choosing thelateral location responsively to the variation in the distribution. 43.The method according to claim 42, wherein choosing the lateral locationcomprises finding the lateral location that minimizes the variation inthe distribution as a function of the vertical location.
 44. The methodaccording to claim 40, wherein adjusting the position comprisesadjusting a skew angle of the knife edge relative to a surface of thesample.
 45. A method for inspection of a sample, comprising: irradiatingthe sample with a beam of X-rays over a range of angles of incidence;positioning a shutter so as to intercept the beam at a predeterminedangle; measuring a distribution of the X-rays that are emitted from thesample responsively to the beam, thereby generating an X-ray spectrum,which includes a shadow of the shutter; determining a tilt angle of thesample responsively to an angular position of the shadow in thespectrum; and processing the spectrum responsively to the tilt angle.46. The method according to claim 45, wherein measuring the distributionof the X-rays comprises measuring the distribution of the X-rays thatare reflected from the sample as a function of the elevation anglerelative to a surface of the sample, and wherein processing the spectrumcomprises calibrating the spectrum with respect to the tilt angle. 47.The method according to claim 46, wherein the predetermined angle isbelow a critical angle of the sample for total external reflection. 48.The method according to claim 45, wherein measuring the distributioncomprises finding a current angular position of the shadow, and whereindetermining the tilt angle comprises comparing the current angularposition to a reference angular position of the shadow, which isindicative of a zero tilt angle.
 49. The method according to claim 48,wherein comparing the current angular position to the reference angularposition comprises finding a difference between the current andreference angular positions, and determining the tilt angle of thesample to be equal to half the difference.
 50. A method for inspectionof a sample, comprising: focusing a beam of X-rays onto a focal locationon the sample; measuring a distribution of the X-rays that are reflectedfrom the sample responsively to the beam, thereby generating an actualreflectance spectrum; estimating a spot size of the beam on the sampleat the focal location; computing a simulated reflectance spectrum of thesample responsively to the spot size; and fitting the simulatedreflectance spectrum to the actual reflectance spectrum in order todetermine one or more properties of the sample.
 51. The method accordingto claim 50, wherein computing the simulated reflectance spectrumcomprises blurring the simulated reflectance spectrum based on anangular spread of the reflected X-rays due to the spot size.
 52. Themethod according to claim 51, wherein estimating the spot size comprisesassessing a variation in an effective spot size of the beam as afunction of an elevation angle relative to the sample, and whereinblurring the simulated reflectance spectrum comprises applying avariable blur to the simulated reflectance spectrum responsively to thevariation in the effective spot size.
 53. The method according to claim50, wherein focusing the beam of X-rays comprises positioning abeam-limiting optic in the beam, and wherein estimating the spot sizecomprises determining the spot size as a function of a position of thebeam-limiting optic relative to a surface of the sample.
 54. The methodaccording to claim 50, wherein fitting the simulated reflectancespectrum to the actual reflectance spectrum comprises determining atleast one of a thickness, a density and a surface quality of a surfacelayer of the sample.
 55. Apparatus for inspection of a sample,comprising: an X-ray source, which is adapted to irradiate the samplewith a beam of X-rays; a detector assembly, which is arranged to measurea distribution of the X-rays that are emitted from the sampleresponsively to the beam, thereby generating an X-ray spectrum; and asignal processor, which is adapted to assess an effect on the spectrumof a non-uniformity of the beam and to correct the spectrum responsivelyto the effect.
 56. The apparatus according to claim 55, wherein thedetector assembly is adapted to measure the distribution of the X-raysthat are reflected from the sample as a function of an elevation anglerelative to a surface of the sample.
 57. The apparatus according toclaim 56, wherein the X-ray source is adapted to focus the beam so thatthe X-rays converge on the sample over a range of incidence angles. 58.The apparatus according to claim 57, wherein the X-ray source is adaptedto focus the beam so as to form a focal spot on the surface of thesample, and wherein the signal processor is adapted to assess avariation in a location of the focal spot.
 59. The apparatus accordingto claim 58, wherein the detector assembly comprises a detector and analignment mechanism, which is adapted to adjust a position of thedetector responsively to the variation in the location of the focalspot.
 60. The apparatus according to claim 59, wherein the alignmentmechanism is adapted to align the detector so as to maintain a constantdistance between the focal spot and the detector notwithstanding thevariation in the location of the focal spot.
 61. The apparatus accordingto claim 58, and comprising a beam-limiting optic, which is arranged tobe introduced into the beam, wherein the signal processor is adapted toassess the variation in the location due to introduction of thebeam-limiting optic.
 62. The apparatus according to claim 61, whereinthe beam-limiting optic comprises a knife edge, which is arranged tointercept the beam in a position adjacent to the focal spot.
 63. Theapparatus according to claim 62, and comprising a positioning assembly,which is adapted to adjust a position of the knife edge, and wherein thesignal processor is adapted to assess the variation as a function of theposition of the knife edge.
 64. The apparatus according to claim 63,wherein the positioning assembly is adapted to adjust a height of theknife edge relative to the surface of the sample, and wherein the signalprocessor is adapted to determine a correction to apply to the spectrumresponsively to the height.
 65. The apparatus according to claim 63,wherein the positioning assembly is adapted to adjust a lateral positionof the knife edge relative to the beam, and wherein the signal processoris adapted to select the lateral position so as to minimize the effectof the non-uniformity.
 66. The apparatus according to claim 58, whereinthe signal processor is adapted to record measured values of intensityof the X-rays at a function of the elevation angle, and to correct themeasured values to account for the variation in the location of thefocal spot.
 67. The apparatus according to claim 66, wherein the signalprocessor is adapted to determine an effective variation in the locationof the focal spot as a function of the elevation angle, and to adjust amapping of the measurement values to elevation angles responsively tothe effective variation.
 68. The apparatus according to claim 56, andcomprising a beam-limiting optic, which is arranged to be introducedinto the beam, wherein the signal processor is adapted to assess avariation in the beam as a function of the elevation angle due tointroduction of the beam-limiting optic.
 69. The apparatus according toclaim 68, wherein the X-ray source is adapted to direct the beam toimpinge on the sample at a focal location, and wherein the beam-limitingoptic comprises a knife edge, which is arranged to intercept the beam ina position adjacent to the focal location.
 70. The apparatus accordingto claim 69, and comprising a positioning assembly, which is adapted toadjust a height of the knife edge relative to the surface of the sample,and wherein the signal processor is adapted to apply a correction to thespectrum responsively to the height.
 71. The apparatus according toclaim 69, and comprising a positioning assembly, which is adapted toadjust a lateral position of the knife edge relative to the beam, andwherein the signal processor is adapted to select the lateral positionso as to minimize the effect of the non-uniformity.
 72. The apparatusaccording to claim 68, wherein the signal processor is adapted todetermine a correction factor as a function of the elevation angleresponsively to the variation, and to apply the correction factor inorder to correct to the spectrum.
 73. The apparatus according to claim72, wherein the detector assembly comprises a detector array, whichcomprises a plurality of detector elements, and wherein the signalprocessor is adapted to determine the correction factor by comparing afirst measurement of a flux of the X-rays that is incident on each ofthe detector elements with the beam-limiting optic in a first positionwith a second measurement of the flux of the X-rays that is incident oneach of the detector elements with the beam-limiting optic in a secondposition.
 74. The apparatus according to claim 73, wherein the detectorarray is arranged to measure the distribution of the X-rays that arereflected from the sample with the beam-limiting optic in the firstposition.
 75. The apparatus according to claim 74, wherein the sample isremoved from the beam of X-rays while the first and second measurementsare made so that the beam is directly incident on the detector array.76. The apparatus according to claim 74, wherein a reflective surface isintroduced into the beam at a location of the sample while the first andsecond measurements are made so that the X-rays are reflected onto thedetector array.
 77. The apparatus according to claim 55, and comprisinga beam-limiting optic, which is arranged to be introduced into the beam,wherein the signal processor is adapted to measure a variation in thebeam due to introduction of the beam-limiting optic.
 78. The apparatusaccording to claim 77, wherein the signal processor is adapted todetermine a correction vector responsively to the variation, and tocorrect the spectrum by applying the correction vector to the spectrum.79. The apparatus according to claim 78, wherein the detector assemblycomprises a detector array, which comprises a plurality of detectorelements, and wherein the signal processor is adapted to determine thecorrection vector by comparing a first measurement of a flux of theX-rays that is incident on each of the detector elements with thebeam-limiting optic in a first position with a second measurement of theflux of the X-rays that is incident on each of the detector elementswith the beam-limiting optic in a second position.
 80. The apparatusaccording to claim 79, wherein the detector array has an axis, andwherein the detector assembly comprises an alignment mechanism, which isadapted to rotate the array so as to position the axis perpendicular toa surface of the sample.
 81. The apparatus according to claim 55,wherein the signal processor is adapted to receive a tilt map indicatinga characteristic tilt angle of the sample at multiple points over asurface of the sample, and to determine, based on the tilt map, a localtilt angle at a location on the sample upon which the beam of X-rays isincident, and to correct the spectrum responsively to the local tiltangle.
 82. Apparatus for inspection of a sample, comprising: an X-raysource, which is adapted to irradiate a surface of the sample with abeam of X-rays; a detector array, which has an axis and comprises aplurality of detector elements arranged along the axis, and which isarranged to measure a distribution of the X-rays that are emitted fromthe sample responsively to the beam; and an alignment mechanism, whichis coupled to rotate the detector array so as to position the axisperpendicular to the surface of the sample.
 83. The apparatus accordingto claim 82, wherein the detector array is arranged to measure thedistribution of the X-rays that are reflected from the sample as afunction of an elevation angle relative to a surface of the sample. 84.The apparatus according to claim 83, wherein the distribution of theX-rays comprises an oscillatory pattern as a function of the elevationangle, and comprising a signal processor, which is adapted to controlthe alignment mechanism so as to rotate the detector array responsivelyto the oscillatory pattern.
 85. The apparatus according to claim 84,wherein the signal processor is adapted to cause the alignment mechanismto rotate the detector array so as to maximize a contrast of theoscillatory pattern.
 86. The apparatus according to claim 82, whereinthe X-ray source is adapted to direct the beam toward the surface at agrazing incidence, and comprising a signal processor, which is adaptedto detect a transition in the distribution corresponding to a plane ofthe surface of the sample, and to control the alignment mechanism so asto rotate the detector array responsively to the transition.
 87. Theapparatus according to claim 86, wherein the signal processor is adaptedto cause the alignment mechanism to rotate the detector array so as tomaximize a sharpness of the transition.
 88. Apparatus for inspection ofa sample, comprising: an X-ray source, which is adapted to irradiate asample with a beam of X-rays; a detector, which is adapted to receivethe X-rays reflected from a location on the sample so as to acquire anX-ray reflectance (XRR) spectrum of the sample at the location; and asignal processor, which is adapted to receive a tilt map, indicating acharacteristic tilt angle of a reference surface at multiple points onthe reference surface, and to determine an estimated tilt angle of thefirst location on the sample based on the tilt map, and to correct theXRR spectrum responsively to the estimated tilt angle.
 89. The apparatusaccording to claim 88, wherein the tilt map is produced by measuring thecharacteristic tilt angle of the reference surface at each of themultiple points.
 90. The apparatus according to claim 89, and comprisinga mounting assembly, on which the sample is mounted while acquiring theXRR spectrum, and wherein the characteristic tilt angle at each of themultiple points is measured by mounting the reference surface on themounting assembly, and measuring the characteristic tilt angle while thereference surface is on the mounting assembly.
 91. The apparatusaccording to claim 90, wherein the mounting assembly is adapted torotate and translate the sample, and wherein the tilt. map is indicativeof the tilt angle as a function of rotation and translation of thesample.
 92. The apparatus according to claim 88, wherein the signalprocessor is adapted to find the estimated tilt angle by interpolationalong a curve in the tilt map.
 93. The apparatus according to claim 88,wherein the sample comprises a semiconductor wafer.
 94. Apparatus forinspection of a sample, comprising: an X-ray source, which is adapted tofocus a beam of X-rays onto a focal location on the sample; a knifeedge; a positioning assembly, which is adapted to position the knifeedge so as to intercept the beam in a position adjacent to the focallocation; a detector assembly, which is arranged to measure adistribution of the X-rays that are emitted from the sample, therebygenerating an X-ray spectrum; and a signal processor, which is adaptedto receive the X-ray spectrum, and to assess an effect of the positionof the knife edge on the X-ray spectrum, and to cause the positioningassembly to adjust the position of the knife edge responsively to theeffect.
 95. The apparatus according to claim 94, wherein the positioningassembly is adapted to adjust a lateral location of the knife edgerelative to the beam under control of the signal processor.
 96. Theapparatus according to claim 95, wherein the signal processor is adaptedto assess a variation in the distribution of the X-rays as a function ofa vertical location of the knife edge relative to a surface of thesample at each of a plurality of lateral locations of the knife edge,and to choose the lateral location responsively to the variation in thedistribution.
 97. The apparatus according to claim 96, wherein thesignal processor is adapted to choose the lateral location thatminimizes the variation in the distribution as a function of thevertical location.
 98. The apparatus according to claim 94, wherein thepositioning assembly is adapted to adjust a skew angle of the knife edgerelative to a surface of the sample under control of the signalprocessor.
 99. Apparatus for inspection of a sample, comprising: anX-ray source, which is adapted to irradiate the sample with a beam ofX-rays over a range of angles of incidence; a shutter, which ispositioned so as to intercept the beam at a predetermined angle; adetector assembly, which is arranged to measure a distribution of theX-rays that are emitted from the sample responsively to the beam,thereby generating an X-ray spectrum, which includes a shadow of theshutter; and a signal processor, which is adapted to determine a tiltangle of the sample responsively to an angular position of the shadow inthe spectrum, and to process the spectrum responsively to the tiltangle.
 100. The apparatus according to claim 99, wherein the detectorassembly is arranged to measure the distribution of the X-rays that arereflected from the sample as a function of the elevation angle relativeto a surface of the sample, and wherein the signal processor is adaptedto calibrate the spectrum with respect to the tilt angle.
 101. Theapparatus according to claim 100, wherein the predetermined angle isbelow a critical angle of the sample for total external reflection. 102.The apparatus according to claim 99, wherein the signal processor isadapted to find a current angular position of the shadow in thespectrum, and to determine the tilt angle by comparing the currentangular position to a reference angular position of the shadow, which isindicative of a zero tilt angle.
 103. The apparatus according to claim102, wherein the signal processor is adapted to find a differencebetween the current and reference angular positions, and to determinethe tilt angle of the sample to be equal to half the difference. 104.Apparatus for inspection of a sample, comprising: an X-ray source, whichis adapted to focus a beam of X-rays onto a focal location on thesample; a detector assembly, which is adapted to measure a distributionof the X-rays that are reflected from the sample responsively to thebeam, thereby generating an actual reflectance spectrum; and a signalprocessor, which is adapted to estimate a spot size of the beam on thesample at the focal location, to compute a simulated reflectancespectrum of the sample responsively to the spot size, and to fit thesimulated reflectance spectrum to the actual reflectance spectrum inorder to determine one or more properties of the sample.
 105. Theapparatus according to claim 104, wherein the signal processor isadapted to blur the simulated reflectance spectrum based on an angularspread of the reflected X-rays due to the spot size.
 106. The apparatusaccording to claim 105, wherein the signal processor is adapted toassess a variation in an effective spot size of the beam as a functionof an elevation angle relative to the sample, and to apply a variableblur to the simulated reflectance spectrum responsively to the variationin the effective spot size.
 107. The apparatus according to claim 104,and comprising a beam-limiting optic, which is positioned in the beam ofthe X-rays, wherein the signal processor is adapted to estimate the spotsize as a function of a position of the beam-limiting optic relative toa surface of the sample.
 108. The apparatus according to claim 104,wherein the signal processor is adapted to fit the simulated reflectancespectrum to the actual reflectance spectrum so as to determine at leastone of a thickness, a density and a surface quality of a surface layerof the sample.
 109. A cluster tool for producing microelectronicdevices, comprising: a deposition station, which is adapted to form athin-film layer on a surface of a semiconductor wafer; and an inspectionstation, comprising: an X-ray source, which is adapted to irradiate thesemiconductor wafer with a beam of X-rays; a detector assembly, which isarranged to measure a distribution of the X-rays that are emitted fromthe semiconductor wafer responsively to the beam, thereby generating anX-ray spectrum; and a signal processor, which is adapted to assess aneffect on the spectrum of a non-uniformity of the beam and to correctthe spectrum responsively to the effect.
 110. Apparatus for producingmicroelectronic devices, comprising: a production chamber, which isadapted to receive a semiconductor wafer; a deposition device, which isadapted to deposit a thin-film layer on a surface of the semiconductorwafer within the chamber; an X-ray source, which is adapted to irradiatethe semiconductor wafer in the production chamber with a beam of X-rays;a detector assembly, which is arranged to measure a distribution of theX-rays that are emitted from the semiconductor wafer responsively to thebeam, thereby generating an X-ray spectrum; and a signal processor,which is adapted to assess an effect on the spectrum of a non-uniformityof the beam and to correct the spectrum responsively to the effect.